Known shaft seals are, for example, labyrinth seals. A leakage flow is in this case braked by what are known as sealing lips. Seals of this type are employed in various turbomachines.
The collective designation “turbomachine” embraces water turbines, steam and gas turbines, wind wheels, centrifugal pumps and centrifugal compressors and also propellers. All these machines have in common the fact that they serve the purpose of extracting energy from a fluid in order thereby to drive another machine or, conversely, to supply energy to a fluid in order to increase its pressure. A fluid is to be understood as meaning both liquids and gases which can execute flowing movements.
As in other thermal turbomachines, the shaft seals through the casing and the inner sealing points of the machines are conventionally sealed off by means of labyrinth seals. In the case of combustible, toxic or costly gases, however, it is necessary to have with respect to the surroundings a complete gastight closure which cannot be achieved by means of the labyrinth seals.
This may be remedied by floating ring seals or seals with a liquid barrier. In such arrangements, sealing off takes place by means of a liquid film between a shaft and the bore of sealing rings which are secured against rotation, but can move in the radial direction, so that they can follow the displacements of the shaft with respect to the casing. The barrier liquid is supplied between two rings and emerges on the gas and atmosphere side. Oils and water are primarily considered as barrier liquids. Selection must take into account the variations which the liquid may experience due to dissolved gases, such as foaming, viscosity changes or chemical reactions.
Of course, the sealing action is the better, the narrower the gap between the rotor and the stator is. It is desirable that such sealing arrangements be designed in such a way that an easy mountability of the overall turbomachine can be carried out. Furthermore, it is necessary to ensure that the rotor is rotatable in the stator under all operating conditions. The markedness of an undesirable gap between a stator and a rotor is influenced by centrifugal forces and pressure differences and also different expansions of the rotor and stator on account of high temperature fluctuations.
Seals in which a sealing action is achieved by means of barrier gas are known. High barrier gas consumptions sometimes have to be allowed for in order to ensure a good sealing action. Moreover, seals are known in which there is contact between the stator and the rotor. Brush seals, as they are known, whose sealing action is afforded even in the case of varying gap widths, are known in this context.